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Radionuclide retention in claystones and clay minerals under hyperalkaline conditions.

Ph.D. student:

Thimo Philipp

Supervisor:

Prof. Dr. Thorsten Stumpf, Dr. Katja Schmeide (HZDR)

Division:

Surface processes

Period:

12/2015–11/2018

In the site model NORD, developed as part of the AnSichT-Project, held by GRS, BGR and DBE, claystone is considered as a potential host rock for the deep geological disposal of radioactive waste. Also in the geotechnical barrier a utilization of clay, in the form of bentonite, is planned. In addition, clay minerals can form as alteration products within the disposal zone, for example hectorite as a result of corroded borosilicate glass. Therefore profound understanding about the radionuclide retention processes in claystones and clay minerals is essential for the long-term safety assessment of a radioactive waste repository.

The pore water chemistry of Northern German clay formations is characterized by intermediate to high ionic strengths, promoting the corrosion of concrete, which is part of the engineered barrier. This leads to a considerable modification of the geochemical environment in the near field of a hypothetical repository. Upon corrosion, hyperalkaline (10 < pH < 13) cement pore waters form, which, in turn, interact with the bentonite buffer and the host rock, to ultimately alter the radionuclide retention potential of those argillaceous media. In order to reproduce the in-situ conditions upon water ingress, experiments regarding the immobilization or mobilization of radionuclides are conducted under hyperalkaline conditions and at intermediate to high ionic strengths within the present research project. Up until a few years ago, research focused mainly on radionuclide retention at lower ionic strengths and pH or on single clay mineral phases.

In the course of the doctoral studies different retention mechanisms shall be considered. Those include the adsorption of radionuclides on clay mineral surfaces as well as structural incorporation into the mineral phases (absorption). The following work-packages will be approached:

Batch-experiments to study the uranium sorption on Ca-Bentonite and Opalinus Clay

Diffusion of uranium in Ca-Bentonite and Opalinus Clay

Spectroscopic investigation of the stability of radionuclide-doped hectorite under variable experimental conditions

This work is part of the joint project: "Geochemische Radionuklidrückhaltung an Zementalterationsphasen" (GRaZ).